Lecture 1 Notes — Blood Constituents, Formed Elements, and Key Physiology

Blood Constituents and Components

  • Based on Lecture 1 slides (BPK305, Drs. Wicks & Claydon)
  • Focus: major constituents of blood, formed elements, hematopoiesis regulation, and key functional concepts related to oxygen transport and clotting.

Plasma Constituents

  • Overall composition of plasma/serum:
    • Electrolytes: Na⁺ ≈ 145 mM; K⁺ ≈ 4–5 mM; Ca²⁺ ≈ 2–2.5 mM; Mg²⁺ ≈ 1.5 mM; H⁺ (pH) ≈ 7.35–7.45; HCO₃⁻ ≈ 24 mM.
    • Proteins (~7% w/v): albumin ≈ 4.2 g/100 mL; globulin ≈ 2.8 g/100 mL; fibrinogen ≈ 0.3 g/100 mL.
    • Gases: CO₂, O₂, N₂.
    • Nutrients: glucose ≈ 5 mM; lipids ≈ 7.5 mM; cholesterol ≈ 4–7 mM; vitamins < 0.1 mM; free fatty acids ≈ 0.4–2 mM.
    • Waste products: urea, creatinine, uric acid, bilirubin.
    • Water: ≈ 92% w/v.
  • Additional notes:
    • Plasma contains varied solutes to support cellular metabolism, transport, and buffering.
    • Some values are given as approximate muscular/physiological ranges and can vary with diet, hydration, and health status.

Formed Elements (Blood Cells)

  • Overall composition:
    • Red blood cells (RBCs, erythrocytes): ~5 million per μL.
    • White blood cells (WBCs, leukocytes): ~7,000 per μL.
    • Platelets (thrombocytes): ~250,000 per μL.
    • Together, form ~40–45% of blood volume (hematocrit).
  • Centrifuged blood components:
    • Plasma on top, buffy coat (WBCs + platelets) in middle, packed RBCs at bottom.
    • Hematocrit = Height of RBCs / Total height.
  • Formed elements lifespans and turnover (Hematopoiesis):
    • Leukocytes (WBCs): produced predominantly as 75% of new cells; lifespan hours to days.
    • Erythrocytes (RBCs): produced ≈ 20–25% of new cells; lifespan ≈ 90–120 days.
    • Platelets: produced via thrombopoiesis (see below).
  • Site and regulation of hematopoiesis:
    • Occurs in bone marrow.
    • Cytokines regulate hematopoiesis:
    • Colony Stimulating Factors → leukocytes.
    • EPO (erythropoietin) → erythrocytes.
    • TPO (thrombopoietin) → platelets.
    • Macrophages and mast cells are part of the hematopoietic milieu; white blood cells are continually replaced.

Hematopoiesis and Regulation (Expanded)

  • Key regulators:
    • Colony Stimulating Factors (CSFs) drive leukocyte formation.
    • Erythropoietin (EPO) stimulates erythrocyte production.
    • Thrombopoietin (TPO) stimulates platelet production.
  • Hematopoiesis occurs in the bone marrow; ongoing turnover of immune cells and red cell lineage ensures maintenance of blood cell populations.

Hemoglobin (Hb): Structure, Function, and Regulation

  • Hb subunit composition:
    • Hb has 4 globin subunits.
    • Each subunit contains a heme group with an iron (Fe) atom that binds oxygen.
  • Oxygen binding capacity:
    • Hb can bind up to 4 O₂ molecules (one per heme group).
    • Binding is cooperative: binding of one O₂ increases the affinity of remaining sites for O₂.
    • O₂ binding capacity can be summarized as Hb + 4 O₂ ⇌ Hb(O₂)₄.
  • Functional roles and regulation:
    • Primary role: O₂ transport from lungs to tissues.
    • Hb saturates with O₂ when O₂ tension is high; releases O₂ when O₂ tension is low.
    • Hb surface antigens (A, B, Rh) are clinically important for transfusion compatibility and rejection.
  • Hemoglobin synthesis prerequisites:
    • Iron (Fe), vitamin B₁₂, and folic acid are required for Hb production.
  • Key concept: Hb can load up when O₂ levels are high and unload when O₂ levels are low; the loading/unloading is regulated by tissue oxygen demand and environmental conditions.

Oxygen Transport Physiology and Hemoglobin Affinity

  • Oxygen dissociation curve and pO₂:
    • The curve is sigmoidal due to cooperative binding.
    • P₅₀ (P50) is defined as the PO₂ at which Hb is 50% saturated with O₂.
    • Typical description: Hb affinity for O₂ is modulated by pO₂, pH, temperature, CO₂, and allosteric effectors.
  • pO₂ values in physiological contexts (described conceptually):
    • Capillaries in active muscle have lower pO₂ than alveoli in lungs.
    • The P₅₀ value lies between these two physiological pO₂ ranges, reflecting a balance between loading in the lungs and unloading in tissues.
  • Practical takeaway:
    • Higher tissue activity lowers local pO₂, promoting O₂ release from Hb.
    • In the lungs, high pO₂ promotes Hb loading.

Red Blood Cell Hematocrit and Anemia

  • Hematocrit determinants:
    • Lower hematocrit is seen in women compared to men; altitude training or living at altitude can increase hematocrit; athletes may have higher hematocrit.
    • Regulation is influenced by oxygen availability via EPO, nutritional status, menstruation/hemorrhage, hormones, and vitamins (notably B12 and folic acid).
  • Anemias (types related to Hb and RBC production):
    • Hypochromic anemia: low Hb content in RBCs, often due to iron deficiency.
    • Megaloblastic anemia: due to B12 or folate deficiency (pernicious or non-pernicious causes).
    • Hemolytic anemia: RBCs are fragile and destroyed prematurely.
    • Aplastic anemia: reduced RBC production due to bone marrow failure.
  • Conceptual equation:
    • Hematocrit ≈
      (height of RBC column) / (total blood height),
      and low hematocrit reflects reduced RBC mass or iron/nutritional deficiency.

Bilirubin, Hb Metabolism, and Jaundice

  • Hyperbilirubinemia and jaundice:
    • Excess bilirubin accumulation can cause jaundice.
    • Hyperbilirubinemia can be related to impaired Hb metabolism, increased hemolysis, or immature bilirubin processing.
  • Bilirubin handling involves hepatic processing and urinary excretion; shifts in bilirubin metabolism are seen in neonatal periods (e.g., 3–9 days old) and can be clinically relevant.

Clotting, Platelets, and Hemostasis

  • Endothelial injury response:
    • Exposure of collagen at damaged vasculature promotes platelet activation.
    • Prostacyclin production is reduced at injury sites, allowing platelets to adhere and aggregate.
  • Platelet products released during activation:
    • Serotonin (5-HT), ATP, Thromboxane A2 contribute to vasoconstriction and aggregation.
  • Coagulation cascade components (brief overview):
    • Platelets and fibrinogen participate in forming a platelet plug and fibrin mesh.
    • Fibrin formation is mediated by thrombin converting fibrinogen to fibrin.
    • Other clotting factors I–XIII participate in a proteolytic cascade that stabilizes the clot (Prothrombin is converted to thrombin; thrombin then acts on fibrinogen to form fibrin).
  • Summary of clot formation:
    • Platelet adhesion and aggregation at the site of injury, followed by activation of coagulation factors and conversion of fibrinogen to fibrin to stabilize the clot.

White Blood Cells (Leukocytes): Types and Roles

  • Granulocytes (PMNs):
    • Neutrophils: 50–70% of WBCs; primary defense against bacteria; rapid responders.
    • Eosinophils: destroy invading parasites and modulate allergic responses.
    • Basophils: form mast cells; release histamine; participate in allergic reactions and inflammation.
  • Monocytes and macrophages:
    • Monocytes differentiate into macrophages; phagocytose invaders; antigen presentation.
  • Lymphocytes:
    • B cells (humoral immunity) produce antibodies.
    • T cells (cell-mediated immunity).
    • Natural killer (NK) cells provide innate immune responses.
  • Summary:
    • White blood cells are diverse in function, with granulocytes handling immediate defense and monocytes/macrophages and lymphocytes handling antigen presentation and adaptive immunity.

Antigens on Red Blood Cells and Transfusion Relevance

  • RBC surface antigens:
    • A, B antigens and Rh factor (D antigen) are critical for transfusion compatibility.
    • Mismatches can lead to transfusion reactions and immune rejection.

Practical Implications and Connections

  • Real-world relevance:
    • Hematocrit and Hb levels are routinely used in medical diagnostics to assess anemia, hydration status, and overall oxygen-carrying capacity.
    • EPO, iron, B12, and folate status are often evaluated in patients with anemia to identify underlying causes.
    • Neonatal jaundice relates to bilirubin metabolism; management depends on bilirubin levels and maturation of hepatic pathways.
    • Blood typing and transfusion medicine rely on RBC antigen compatibility to prevent transfusion reactions.
  • Ethical and practical considerations:
    • Blood donation and transfusion require careful matching and monitoring for adverse reactions.
    • Resource allocation and blood safety are ongoing ethical concerns in clinical practice.

Key Formulas and Notation (LaTeX)

  • Hematocrit definition:
    \text{Hematocrit} = \frac{\text{Height of RBCs}}{\text{Total height}}.
  • Hemoglobin binding to oxygen (cooperative binding):
    \mathrm{Hb} + 4\,\mathrm{O}2 \rightleftharpoons \mathrm{Hb}(\mathrm{O}2)_4.
  • Oxygen transport concepts (P50):
    P{50} = \text{PO}2 \text{ at which Hb is 50\% saturated}.
  • Hb oxygen binding capacity (qualitative): Hb can bind up to 4 O₂ molecules via its four heme groups with cooperative binding.
  • Typical RBC, WBC, and platelet counts (reference ranges):
    • RBCs ≈ $5 \times 10^6$ / µL
    • WBCs ≈ $7 \times 10^3$ / µL
    • Platelets ≈ $2.5 \times 10^5$ / µL

Notes on Sources and Format

  • These notes follow the sequence and content of the provided lecture transcript slides (L01) and compress key quantitative and qualitative concepts for study purposes.
  • For exam prep, focus on understanding the relationships between hematocrit, oxygen transport, and the regulation of hematopoiesis; as well as the coagulation cascade and leukocyte diversity.